I have inherited a solution created in ASP.net with VB.net and am having a real problem with the error message "Content controls have to be top-level controls in a content page or a nested master page that references a master page."

The design is somewhat complex as follows:
The app uses a single Masterpage. The child page accessed has a custom user control on it. The user control contains, amongst other controls, an iframe. The source of the iframe is set programatically to another page, (aspx) which does not inherit from the Masterpage as we don't want the headers, footers etc. to appear in the iframe.
This aspx page has another custom user control on it that displays a small number of controls, buttons, text fields, file upload etc.)

This all works well in my IDE (Visual Studio 2005) and the file upload control is displayed properly. As soon as it's deployed to a server, (test or live) I get the error message at the beginning of this post.

The pages and code are too complex to post here but if I haven't given enough detail I will try to explain as necessary. I hope someone can help as this has been driving me insane. I've searched extensively with no success.